Coating cupreous surfaces with tin



4 Patented May 12, 1942 iii" COATING CUPREOUS SURFACES WITH TIN .lohn Clement Bradley, Waterbury, Conn assignor to The American Brass Company, a corporation of Connecticut No Drawing. Application March 20, 1940, Serial No. 324,915

7 Claims.

This invention relates to coating cupreous surfaces with tin, and has for its principal object the provision of an improved process for depositing a coating of metallic tin on such surfaces.

Copper is well-known for its resistance to corrosion', but when a copper article is exposed to moisture a film or patina of green-colored copper compounds forms on its surface. For many purpose this green patina is not objectionable, but in some cases it is undesirable. For example, when such compounds form inside copper water pipes, the water delivered through the pipes is likely to be tinged with green. This slight coloring of the water gives it an objectionable appearance if the water is intended for human consumption or for other domestic uses. a

It is known that a coating of tin applied t the surface of copper or other cupreous articles protects the underlying cupreous surface from the formation of green-colored copper compounds. The present invention provides an improved process for coating cupreous surfaces with tin. The process of the invention is an immersion or bathing process which does not involve the use of an electric current, and by means of which it is possible to apply thin, continuous coatings of tin of uniform thickness to the inside surfaces of cupreous water pipes, or to other intricate or relatively inaccessible cupreous surfaces, The process of the invention involves bathing the surface to be coated with an aqueous solution containing a dissolved tin compound and dissolved thiocarbamide. As a result of bathing the surface with such a solution, a thin, continuous adherent deposit of metallic tin is formed directly on the cupreous surface. The process may be employed eifectively for coating the interior surfaces of cupreous water pipes with tin by simply passing the solution through the pipes.

Among articles having a cupreous surface which may be coated in accordance with the invention may be mentioned articles of copper or of copper alloys, such for example as brass, and articles composed mainly of some other metal than copper but having a surface layer of copper or of a copper alloy, such for example as copperor brass-plated articles.

It is possible to obtain a deposit of metallic tin on cupreous articles using a solution in accordance with the invention which contains almost any concentration of dissolved tin and almost any concentration of dissolved thiocarbamide. Generally, however, to obtain satisfactory tin deposits in a reasonable period of time the concentration of thiocarbamide in the solution should be upwards of about 50 grams per liter. Solutions containing about grams per liter of dissolved thiocarbamide are especially satisfactory for producing smooth adherent deposits of tin of satisfactory thickness on the article to be coated. The concentration of thiocarbamide may be as high as to grams per liter, but in most cases no particular advantage is obtained by employing solutions of such high concentration, and in fact such high concentrations of thiocarbamide tend to reduce the thickness of the deposited coating of tin.

The concentration of dissolved tin likewise may be varied within wide limits. Coatings may be obtained using a solution containing tin in a concentration corresponding to as little as 1 gram stannous chloride per liter, but for optimum coatings and to avoid depleting the solution of tin too rapidly, it is preferable to employ a solution containing tin in an amount corresponding to at least 5 grams per liter and upwards of stannous chloride. I have obtained highly satisfactory coatings in a minimum period of time using solutions containing tin in an amount corresponding to about 10 grams per liter stannous chloride. If a much smaller amount of tin is employed, the deposit of tin formed on the article being coated may be spongy in character. With materially higher concentration of tin in the solution, the deposit of tin is likely to be thinner than is obtained with approximately this concentration of tin. Satisfactory coating may in some cases be obtained, however, using substantially higher tin concentrations, corersponding for example to 20 grams per liter and even more of stannous chloride.

Preferably a water-soluble stannous salt is employed as the tin compound used in making up the solution. Stannous chloride is a particularly satisfactory tin compound to employ, although other water-soluble stannous salts, such for example as stannous bromide, stannous iodide, and stannous sulphate, also may be used. It is'alsb possible to employ other tin salts such as stannites (sodium stannite, for example) which contain tin in the stannous condition. Stannic salts may be employed, but if used, the solution ad vantageously is of such nature that reduction of the stannic ion to the stannous ion is favored.

It is frequently advantageous to add a small amount of hydrochloric acid to the solution. The presence of hydrochloric acid is not essential, but it appears to facilitate progress of the coating re actions that occur when the solution is employed. Preferably only a small amount of hydrochloric acid is used. About 20 cubic centimeters of concentrated (35%) hydrochloric acid per liter of solution is satisfactory.

The solution may also contain a quantity of an, alkali metal carbonate (such as sodium carbonate) to aid in the formation of a smooth, dense deposit of tin. About grams per liter more or less of sodium carbonate may be employed for this purpose in the solution.

The solution preferably is employed at an elevated temperature upwards of about 100 -F. in order to secure rapid coating action. Excellent coatings have been obtained employing the solution at or near its boiling point, but equally satisfactory coatings may be obtained with a solution heated to about 120 F. In general this lower temperature is more economical and therefore preferred.

The thickness of the tin coating formed on the article treated with the solution depends to some extent on the nature of the solution (concentrations of salts, nature of the salts, and temperature of the solution), and also upon the length of time the article to be coated is immersed in the solution or is bathed therewith. In the case of solutions containing preferred concentrations of suitable salts and employed at (35%) hydrochloric acid per liter.

temperatures of about 120 F., coatings of effective thickness are obtained in about one to five minutes time.

The article to be coated should be carefully cleaned before being treated with the solution. The presence of dirt or grease on the cupreous surface interferes with formation of a smooth, continuous and adherent deposit of tin. Cupreous articles may be cleaned, preparatory to treatment, by wayof the cleaning processes wellknown in the manuafacture of brass and copper articles. lution containing nitric acid is a satisfactory cleaning solution for most purposes. If desired, an alkaline cleaning solution may be employed in place of, or in combination with, the acid pickling solution for the purpose removing dirt or grease from the cupreous surface of the article.

Using a solution of the character described, tin deposits are formed on cupreous articles by simply immersing the article in the solution, or by bathing it with the solution, for a suitable period of time. No applied electrical potential is required, the tin coating being formed by an action akin to cementation. The precise mechanism by which the tin coating is formed on the cupreous surface is not fully understood. Experiments indicate, however, that during the course of the coating operation copper is dissolved from the surface of the article being coated and is replaced by tin. The weight of copper dissolved is approximately twice-as great as the weight,

of tin deposited. After the article has been bathed with or immersed in the solution for a sufficient period of time to produce a tin deposit of desired thickness, the article is removed from the solution and may be washed with water. The tin deposit formed is smooth, adherent to the underlying cupreous surface, and continuous, being free of pinholes or other defects. When properly applied, it effectively protects the underlying cupreous surface from formation of green copper compounds, even when the treated article, is exposed to moisture over long periods of time.

As an example of the process of the invention, a solution was prepared containing approxi- For example, a bright dip pickling so-.

mately 90 grams per liter of thiocarbamide and about 10 grams per liter of stannous chloride dissolved in water. The solution was'heatcd to the boiling temperature and a copper article was immersed therein for a period of five minutes. The article was then removed from the solution and washed. A smooth, adherent and continuous tin deposit about 0.00003 inch in thickness was found to have been formed on the surface of the article.

As a further example of the process of the invention, an aqueous solution was prepared containing about 20 grams stannous chloride per liter, about 80 grams thiocarbamide per liter, and about 10 cubic centimeters of concentrated The solution was heated about to the boiling point and a copper article was immersed therein for one minute. Upon removal from the solution, the copper article was found to be coated with a thin, adherent and continuous deposit of tin approximately 0.000026 inch in thickness.

As a further example of the process of the invention, an aqueous solution was prepared containing about 80 grams per liter of thiocarbamide, about 10 grams per liter of stannous chloride, about 20 cubic centimeters of concentrated hydrochloric acid per liter. The solution was heated to about 120 F. and a copper article was immersed therein for a time of one minute. Upon removal of the article from the solution it was found to be coated with a thin, adherent and continuous deposit of tin approximately 0.000031 inch in thickness.

The effect of time of immersion on the thickness of the tin deposit is indicated by the following example: An aqueous solution containing about 80 grams of thiocarbamide per liter, about 10 grams of stannous chloride per liter, and

Time of immersion Thickness of coating Minutes Inch It has been found that excellent coatings of tin may be formed on the inner surfaces of cupreous pipes by passing the solution therethrough at the desired temperature andin such manner that the inner surfaces of the pipe are bathed with the solution for a period of time corresponding to the thickness of the coating desired. The pipes may, if necessary, be cleaned preparatory to such treatment by the methods usually employed for. cleaning cupreous articles. Copper water pipes having their inner surfaces thus tinned deliver water' which is free from the objectionable greenish tinge sometimes noted in water passed through untreated copper pipes or tubes. Even after water has stood in pipes tinned in accordance with the invention for substantial periods of time, it remains undiscolcred.

I claim:

1. The method of coating a cupreous surface with tin which comp 'ises bathing said surface with an aqueous solution containing dissolved tin in the stannous condition, dissolved thiocarbamide, and a small amount of an alkali metal carbonate, whereby a deposit of metallic tin is formed directly on the cupreous surface.

2. The method of coating a cupreous surface with tin which comprises bathing said surface with an aqueous solution containing upwards of about grams per liter of dissolved stannous chloride, upwards of about 50 grams per liter of dissolved thiocarbamide, and a small amount of an alkali metal carbonate, whereby a deposit of metallic tin is formed directly on the, cupreous surface. I

3. The method of coating a cupreous surface with tin which.comprises bathing said surface with an aqueous solution containing about grams per liter of dissolved stannous chloride, about 80 grams per liter of dissolved thiocarbamide, and a small amount of analkali metal carbonate, whereby a deposit of metallic tin is formed directly on the cupreous surface.

4. The method of coating a cupreous surface with tin which comprises bathing said surface with an aqueous solution containing dissolved tin in the stannous condition, upwards of about 50 grams per liter of dissolved thiocarbamide, and a small amount of an alkali metal carbonate,

whereby a deposit of metallic tin is formed directly on the cupreous surface.

5. The method of coating a cupreous surface with tin which comprises bathing said surface with an aqueous solution containing dissolved tin in the stannous condition, upwards of about grams per liter of dissolved thiocarbamide, and a small amount of an alkali metal carbonate at a temperature upwards of about F.-, whereby a deposit of metallic tin is formed directly on thecupreous surface.

6. The method of coating a cupreous surface with tin which comprises bathing said surface with an aqueous solution containing dissolved tin in the stannous condition, upwards of about 50 grams per liter of dissolved thiocarbamide, and a small amount of an alkali metal carbonate at a temperature of about F., whereby a deposit of metallic tin is formed directly on the cupreous surface.

'7. The method of coating the inner surface of a pipe of cupreous material with tin which comprises passing through the pipe an aqueous solution containing dissolved tin in the stannous condition upwards of about 50 grams per liter of dissolved thiocarbamide, and a small amount of an alkali metal carbonate, whereby a deposit of metallic tin is formed directly on the inner surface of said pipe.

JOHN CLEMENT BRADLEY. 

